1. Field of Invention
The present invention pertains to a method for leveling parts in a roller leveling machine.
2. Discussion of Background Art
Roller leveling is a bending method in which certain tools, called leveling rollers, bend the material to be leveled back and forth. The machines used to carry out the method are called leveling machines. In these machines, the material to be leveled is passed between two opposing rows of leveling rollers, which are offset from each other. The upper and lower rows of leveling rollers are offset from each other in such a way that the rollers of one row nest in the intermediate spaces between the opposing rollers. The depth to which the rollers nest in these intermediate spaces and the geometry of the leveling rollers themselves determine the degree of the back-and-forth bending, which must occur in the partially plastic state. The greatest degree of back-and-forth bending occurs on the infeed side of the leveling machine and usually decreases in the direction toward the outfeed side of the machine, wherein as a rule no bending is applied to the leveled material at the outfeed. A distinction is made between strip leveling and part leveling.
Strip leveling is the leveling of sheet metal strips which have been wound up into coils and which must be unwound and flattened before they can be subjected to further processing in a press, for example, or in a profiling system. A strip leveling machine is therefore always an element of a processing line and must ensure the reliability of the process by maintaining the flatness of the leveled strip within the specified tolerances. What is involved here is usually an intermediate fabrication step. Part leveling, however, is usually a final fabrication step.
The thickness spectrum in the case of part leveling machines is much wider than that in the case of strip leveling machines because of the limitation on the thickness of the material which can be wound up into a coil. For an illustration of the part leveling process, see FIG. 1, which shows a schematic diagram of a part leveling machine 1. The part leveling machine 1 comprises an upper leveling roller block 3 and a lower leveling roller block 5. A set of upper leveling rollers 7, supported by backup rollers 8, is mounted in the upper leveling roller block 3. A set of lower leveling rollers 9, supported by backup rollers 10, is mounted in the lower leveling roller block 5. In the case of the part leveling machine 1 shown in FIG. 1, the lower leveling roller block 5 is permanently mounted in a machine stand (not shown), whereas the upper leveling roller block 3 is installed in an upper roller frame so that its position and angle can be adjusted. The leveling gap is adjusted by means of a leveling gap control system, by means of which, for example, deviations from the desired nominal value of the leveling gap can be corrected.
The material to be leveled in the form of a part 11 is conveyed through the part leveling machine 1 in the direction of the arrow 13. It travels from an infeed side 15 to an outfeed side 17. As can be seen in FIG. 1, the upper leveling rollers 7 are nesting in the intermediate spaces between the lower leveling rollers 9, as a result of which the part 11 is bent back and forth. The depth to which the upper leveling rollers 7 nest in the intermediate spaces between the lower leveling rollers 9 decreases continuously in the direction toward the outfeed side 17 until a space is created which is essentially equal to the thickness of the material of the part 11. The leveling gap which has been set between the roller blocks is the factor which determines the result of the leveling process.
Because the machine is designed in blocks, two defined reference points are sufficient to obtain reproducible settings, provided that the leveling rollers are parallel. It is advantageous for the settings to be made near the infeed side and near the outfeed side of the leveling machine. As can be seen in FIG. 1, the upper leveling roller block 3 can be moved vertically, as indicated by the double arrow 19, and it can also be pivoted around an axis parallel to the axes of the upper and lower leveling rollers, as indicated by the double arrow 21. All of the required settings of the leveling gap can thus be realized.
A disadvantage of leveling machines is that the leveling gap does not remain constant during the leveling process; the gap in fact changes in accordance with the elastic behavior of the mechanical components. The thicker the parts, the greater the forces and the larger the necessary dimensions of the components. Especially in cases where the parts to be leveled are thick, the elastic deflections are many times greater than the setting theoretically required for a rigid part.
Installing an automatic leveling gap control system improves the leveling process, i.e., the results of that process, by reducing the effects of the elastic behavior. Sensors, which are attached, for example, to the corners of the upper leveling roller block 3, detect movement, and a control unit actuates a hydraulic or mechanical adjusting element or an adjusting element based on hybrid technology to correct the leveling gap at these points and thus to maintain the set value. The results of the leveling process now depend essentially only on the stiffness of the leveling roller blocks, especially of the upper, movable leveling roller block.
It has been shown that, by the use of leveling gap control, it is possible to level parts in a single pass, that is, parts which in the most favorable case would require several passes through conventional leveling machines or which could not be leveled at all by them.
The leveling process is especially difficult in the case of parts which do not have a rectangular contour but have instead round contours and/or large cut-out areas. To achieve a more-or-less usable end result, it is necessary to conduct extensive practical testing to find the proper settings. This is time-consuming and therefore also expensive.
Many manufacturers today offer the possibility of using the control system to obtain suggestions for the parameter settings by manually entering the thickness of the material, for example, or by acquiring this value from a measuring system. Because the contours of the parts for part leveling can differ widely from each other, as explained above, basing the process on the thickness of the material alone is not sufficient. Readjustment is possible and usually necessary, and the new settings thus found can be stored again.
The crucial point in the leveling of material is that the yield point must be exceeded when the material is bent. The bending moment, that is, the internal load on the material, must be so large that certain parts of the cross section start to flow. The forces required for this are determined by the product bs2σF for a part with a constant rectangular cross section, where b is the width of the material to be leveled, s the thickness of the material, and σF the yield point of the material.
From the field of elastostatics, the variable (bs2)/6, for example, is known as the moment of resistance to bending in the case of a rectangular cross section. The basis of this calculation is beam theory, the essentials of which are shown in FIGS. 2-4. According to this theory, the normal stress σ in the beam cross section is calculated by the relationshipσ=M/Iz where M is the bending moment, I the axial area moment of inertia, and z the distance of the point under consideration from the neutral fiber. In terms of absolute value, the stress, which thus has a linear distribution, is the greatest at the farthest edge point. We therefore obtain:σ=M/W with W=I/|z|max as the moment of resistance. From this we can calculate the bending moment necessary to reach the yield point at the edge of the cross section. In FIG. 2, furthermore, A is the area of the cross section, S the center of gravity, dA a small differential area element, and x-y-z the coordinate system. The latter can also be seen in FIGS. 3 and 4. FIG. 3 shows the change in the bending stress and FIG. 4 the bending moment.
Under the assumption of ideal elastic-plastic material behavior, the greatest bending moment which can be transmitted in the case of a rectangular cross section is 1.5 times the previously mentioned bending moment capable of producing flow at the edge of the cross section. The flow in this case is already distributed over the entire cross section, and the load-bearing capacity of the cross section is exhausted.
The parts to be leveled usually have a cross section which varies as a function of the contour and the thickness of the part. The moment of resistance, therefore, is not constant but rather changes over the length of the part.